A photosensitive glass undergoes change in those areas that are exposed to short wave radiations, such as ultraviolet light. As a result, the exposed areas are capable of changing color when heated. The process of coloration essentially involves producing metal particles in a glass by photoexcitation, followed by thermal treatment.
The phenomenon was initially observed by Dalton in copper-containing glasses as described in U.S. Pat. Nos. 2,326,012 and 2,422,472. Armistead observed the phenomenon in silver-containing glasses as reported in U.S. Pat. No. 2,515,936, and Stookey observed it in gold-containing glasses as reported in U.S. Pat. No. 2,515,937. In the case of metallic silver, the colors observed were predominantly yellow or brown.
It has been conventional wisdom that the presence of boric oxide (B.sub.2 O.sub.3) in a glass composition tends to weaken photosensitivity. It followed, therefore, that B.sub.2 O.sub.3 should be avoided, or at least minimized, in photosensitive glasses. The Armistead patent indicates that up to 10 wt. % B.sub.2 O.sub.3 can be tolerated under certain conditions, but the oxide does not appear in any of the exemplary compositions in the patent. Subsequent patents, including later filed U.S. Pat. Nos. 2,515,943 (Stookey), usually restrict B.sub.2 O.sub.3 to not more than 5%. Consequently, photosensitive glasses have had alkali silicate compositions, that is, they have been composed essentially of alkali metal oxides, divalent metal oxides and silica. In contrast, silver halide photochromic glasses are all based on borosilicate base compositions.
It has long been known that a more varied combination of physical properties can be achieved in borosilicate glasses than can be readily obtained in glasses based on silica alone as the glass former. For example, laboratory ware is usually produced from a borosilicate glass rather than from less expensive lime glasses. This is because the borosilicate is equally easy to melt, has better chemical durability and provides a lower coefficient of thermal expansion.
The restrictions imposed on boric oxide content in the patents cited above, therefore, significantly diminishes the flexibility of the glass chemist in designing photosensitive glasses. This is particularly true of glasses having the physical properties desired for special applications. Accordingly, it would be desirable to have photosensitive glasses available with relatively large B.sub.2 O.sub.3 contents.